Kiev must acknowledge the mass killing of Poles by Ukrainian nationalists in the 1940s for what it is – ethnic cleansing – Polish President Andrzej Duda said after reportedly snubbing an invitation for a joint commemoration.

On Sunday, Duda visited Ukraine to mark the 75th anniversary of the Volhynia massacres, the mass killing of civilians by radical Ukrainian nationalists in what is now western Ukraine. The brutal massacres carried out by the UPA, the militant wing of Stepan Bandera’s Organization of Ukrainian Nationalists (OUN), was part of a larger campaign of ethnic cleansing by Nazi Germany and its allies, but is considered a distinct episode of Ukrainian-Polish ethnic conflict by many historians.

“This day is a sad one for many Polish families, who in 1942-1944 lost their loved ones in Volhynia. It is estimated that 100,000 Poles were murdered here,” Duda said during a visit to Ukraine. “Not soldiers, but ordinary people. Farmers, who cultivated the land, entire families, women, children and elderly people.”

Duda said the killings were “simple ethnic cleansing” ordered by the OUN and aimed at ridding the region of Poles, and that Ukraine has to admit this truth for Ukrainian-Polish relations to move forward.

The difference of opinion on wartime violence is a thorny issue for Warsaw and Kiev. Many in modern Ukraine consider Stepan Bandera a national hero and tend to overlook the atrocities committed by Ukrainian nationalists.

The Ukrainian Institute of National Remembrance insists that the killings in Volhynia and Eastern Galicia were a tragic, though justifiable, response to the persecution of Ukrainians by the Poles, which was not unlike the killings of Ukrainians by Polish militias during the war.

Consistent with this view, Ukrainian President Petro Poroshenko this weekend traveled to Poland to take part in the opening of a memorial to the victims of the March 1944 Sahryn massacre, which was committed by Polish militias, including the Armia Krajowa insurgency force and their allies from the Bataliony Chlopskie.

“In the memory of innocent victims of Sahryn I am calling on all of you, Ukrainians and Poles, for Christian forgiveness. Mutual forgiveness and not revenge is what innocent victims of fratricidal conflicts between our people are calling for from heavens,” Poroshenko said.

Duda also spoke of forgiveness and reconciliation, but noted that the difference in the number of victims on the Polish and Ukrainian sides during the conflict was “glaring.” He said an estimated 5,000 civilians were killed by Polish militias in retaliation for the actions of the Ukrainian nationalists.

As the anniversary of the tragedy was drawing near, Ukraine sought to negotiate a joint commemoration with Poland, which rejected the offer, Ukrainian media reported last week citing diplomatic sources.

Tensions between Kiev and Warsaw have been rising over the past several years, as nationalist sentiments have grown in both nations. The escalation has been marked by several incidents on both sides of the border, including the vandalization of monuments, hostile chants during demonstrations, and in one case a rocket attack on a Polish consulate in Ukraine, which fortunately did not hurt anyone.